Abstract
ABSTRACT Two currently debated problems in galaxy evolution, the fundamentally local or global nature of the main sequence of star formation and the evolution of the mass–size relation of star-forming galaxies (SFGs), are shown to be intimately related to each other. As a preliminary step, a growth function g is defined, which quantifies the differential change in half-mass radius per unit increase in stellar mass (g = d log R1/2/d log M⋆) due to star formation. A general derivation shows that g = KΔ(sSFR)/sSFR, meaning that g is proportional to the relative difference in specific star formation rate between the outer and the inner half of a galaxy, with K a dimensionless structural factor for which handy expressions are provided. As an application, it is shown that galaxies obeying a fundamentally local main sequence also obey, to a good approximation, g ≃ γn, where γ is the slope of the normalized local main sequence ($\mathrm{ sSFR} \,\, \propto \,\, \Sigma _\star ^{-\gamma }$) and n is the Sersic index. An exact expression is also provided. Quantitatively, a fundamentally local main sequence is consistent with SFGs growing along a stationary mass–size relation, but inconsistent with the continuation at z = 0 of evolutionary laws derived at higher z. This demonstrates that either the main sequence is not fundamentally local, or the mass–size relation of SFGs has converged to an equilibrium state at some finite time in the past, or both.
Highlights
How closely is the evolution of star-forming galaxies (SFGs) dictated by large-scale cosmological processes? To what extent are instead SFGs able to regulate their own growth by means of more local mechanisms? These are common questions underlying several ongoing investigations in galaxy evolution and remind us of the importance to establish connections between various sub-fields
In its simplest and most common form, the mass–size relation connects the stellar mass of a galaxy to its half-mass radius R1/2, with the latter often approximated by the half-light or effective radius Reff, measured in the optical or, preferably, near-infrared (e.g. Lange et al 2015; Lelli, McGaugh & Schombert 2016; Wu 2018)
If a local main sequence is a good description of star formation in SFGs at z ∼ 0, the analysis in Section 4.4 indicates a preference for a sub-critical growth function, marginally consistent with critical, g ࣠ gcrit (Fig. 1)
Summary
How closely is the evolution of star-forming galaxies (SFGs) dictated by large-scale cosmological processes? To what extent are instead SFGs able to regulate their own growth by means of more local mechanisms? These are common questions underlying several ongoing investigations in galaxy evolution and remind us of the importance to establish connections between various sub-fields. How closely is the evolution of star-forming galaxies (SFGs) dictated by large-scale cosmological processes? To what extent are instead SFGs able to regulate their own growth by means of more local mechanisms? These are common questions underlying several ongoing investigations in galaxy evolution and remind us of the importance to establish connections between various sub-fields. Two problems in particular are often studied separately, but are – as this study will try to highlight – tightly connected: the local or global nature of the main sequence of star formation and the evolution of the mass–size relation of SFGs
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